We're all familiar with voltage sources from everyday life: batteries, power supplies, and the AC line all provide a fixed voltage regardless of load-current demand—up to the point where they can no longer supply that current at the fixed AC or DC voltage value.
But there's another "source" which is equally important in electronic circuits and systems: the current source. Unlike a voltage source which provides a known voltage, the current source delivers a specified amount of current to the load, regardless of load conditions (again, up to a point where it can no longer do so, of course). Current sources can be designed for a fixed, pre-set output or to deliver a current value that an be electronically varied as needed. Neither the voltage or current source is "better" than the other; they each have their well-defined uses and applications.
Where are current sources used? Among the most-common application is for resistive-type transducers such as RTDs (resistance temperature detectors) where a known current (typically 1 mA or 10 mA) is forced through the sensing element, which is a resistive material (often platinum-based) whose resistance changes in a well-defined relationship with temperature. As the temperature changes, the voltage across the RTD element also changes, and this voltage can be measured with a digital voltmeter or equivalent circuit.
Note that there are also current "sinks" in addition to sources. These serve the same function, but instead of supplying a known amount a current, a current sink accepts a known amount of current. The choice of source versus sink is often related to circuit topology and circuit grounding; it is not related to the concept of current sourcing versus voltage sourcing.
Current sources are also used extensively with LEDs. Unlike traditional incandescent bulbs which need to see a defined voltage across their terminals, LEDs are current-driven components, and their light output is a function of the applied current. Depending on LED type, the current level is typically as low as 10 mA and as high as 50 mA.
Current sources are commonly employed in industrial instrumentation to transit analog or digital information over a long distance, usually using a 4 to 20 mA range. The reason is that the current source – called a current loop in this application – is relatively impervious to noise pickup, a consequence of its low impedance. If a voltage were used instead of a current, there would be two negative results: first, the voltage at the far end would be less than it was at the source due to unavoidable IR drop in the wires; second, the received voltage would be corrupted by noise pickup due to the high-impedance nature of the interconnection.
Finally, current sources are preferred for driving "magnetics" such as relay coils, deflection coils, and similar inductors. This is because the magnetic-field properties of these components are a function of the current through their coil. Certainly, the current can also be determined by the applied voltage, but then the current is the dependent variable rather than the independent one, which is the one which establishes the operating point.
One of the differences between voltage sources and current source is how multiple loads are configured when connected to each. For example, to power multiple 12-V incandescent bulbs, the topology would be a parallel connection of all the bulbs to the 12-V source,. In contrast, multiple LEDs are connected in series, guaranteeing that each LED will receive the same specified current.
At the semiconductor-device level, current sources and current flow are critical to understanding the operation of the standard bipolar transistor. Further, the current relationships within a transistor can be exploited to create a variety of critical circuit functions such a temperature sensing, or exponential and logarithmic analog amplifiers. The simplified Ebers-Moll model of an NPN transistor shows that the emitter current IE is a function of IES, the reverse saturation current of the base–emitter diode (on the order of 10−15 to 10−12 A), base-emitter voltage VBE, and the thermal voltage VT (kT/q, approximately 26 mV at 300 K or room temperature):
IE = IES (eVBE/VT– 1)
This is one fo the key equations used to understand transistor operation in circuit and IC design and analysis.
Compliance voltage
In the example of the series string of LEDs driven by a single current source, it may seem that there is no limit to the number of LEDs which can be can be connected, as they will all see the same current value. In theory, this may be true, but in practice there is a limit.
The reason is that there is an inherent forward voltage drop across each LED, typically between 1.5 V to 3 V depending on LED type and color. These voltage drops add up and define the compliance voltage which the current source must be able to reach as it provides the current. For a string of ten 2-V LEDs, the compliance voltage is 20 V, so the current source must be able to deliver the 20 mA at that voltage.
When using a current source, it's important to determine what the voltage across the current-driven load will be (whether due to diode drops or voltage drops as current flows through a resistive lord) and ensure that the source can provide the current at the compliance voltage. Compliance voltage is the complement of the situation of a voltage source and loads in parallel: the voltage source can provide the fixed voltage to multiple light bulbs, but at some point it can no longer provide the sum of the currents needed by the bulb array.
Building a current source
There are many ways to build a current source. The simplest way is to use a voltage source, such as a supply rail or battery in series with a resistor. The size of the resistor is calculated to limit the current from the voltage source to the desired value. This is a viable, low-cost approach, as long as the load resistance is fixed, the source voltage is constant, or the current-source value is not critical.
However, many designs need a constant current value, regardless of those factors. For these situations, a basic current source can be built using a single bipolar transistor. The simple gain equation of the transistor shows that the collector-emitter current is equal to the base-emitter current times the transistor gain, a factor which is usually between ×50 and ×1000 depending on the transistor version. Therefore, by driving a small, known current into the transistor base and biasing the transistor into its active region, the collector current is a viable current source.
This simple circuit will work and is used in low-cost, low-performance applications, but suffers from temperature drift and variations in output due to other component-dependent factors. For this reason, most engineers who need a better current source choose an IC version from one of the many vendors who offer them. These ICs, which are the current complement to the ubiquitous low dropout (LDO) voltage regulator, provide an accurate, consistent current source at a fixed-in-advance value or a user-settable value, depending on the device selected.
The LT3092 from Linear Technology Corp. is one example. This two-terminal programmable-current source can provide up to 200 mA, with the specific current value set by user-supplied pair of resistors pair. It operates from a1.2 V to 40 V source and provides 1% accuracy over the operating temperature range.
Current sources may be less intuitive or well known than voltage sources, but they are an important and essential aspect of electronic circuit design. Fortunately, they are no more difficult to implement than voltage sources, but do require a shift in thinking about the independent and dependent relation between voltage and current.